The invention relates to a method for coupling at least one secondary energy source to a power supply system and a power supply system, particularly a vehicle onboard power supply system, to which a load and a primary energy source are connected.
Need-based coupling of various energy sources to a load during the operation thereof generally requires at least one current regulator in order to avoid high compensating currents or switching currents at the moment at which an unloaded, secondary energy source is connected to a primary energy source loaded by the load. Such a situation exists in a vehicle onboard power supply system, for example, which includes a primary energy source, for example a fuel cell or a high-voltage battery, that supplies power to the loads connected to the onboard power supply system. By way of example, such a load may be an inverter that supplies power to an electric (drive) machine as a consumer. The current regulator normally used in such a vehicle onboard power supply system is a DC/DC converter. The task of a DC/DC converter is to use current or voltage regulation to avoid the aforementioned compensating currents in order to prevent destruction, e.g. of an intermediate circuit capacitor that is connected to an input of the inverter.
However, a DC/DC converter, which is used particularly in the high-power drive domain, entails substantial costs and also has a high weight and requires a large installation space. In addition, there is the problem that the operation of a DC/DC converter involves losses of efficiency that reduce energy efficiency, particularly in an electrically operated vehicle.
It is an object of the present invention to provide methods that allow connection and disconnection, i.e. coupling and decoupling, of a secondary energy source to/from a power supply system, particularly a vehicle onboard power supply system, with reduced losses and, at the same time, minimized design complexity. It is also an object of the present invention to provide a power supply system, particularly a vehicle onboard power supply system, that can be operated with minimal losses when a plurality of energy sources are used.
These and other objects are achieved by methods and a power supply system according to embodiments of the invention.
A method for coupling at least one secondary energy source to a power supply system, particularly a vehicle onboard power supply system, to which a load and a primary energy source having a first voltage characteristic are connected, is provided. The at least one secondary energy source has a respective second voltage characteristic that has an operating voltage range that overlaps the first voltage characteristic. The primary energy source and the at least one secondary energy source can be connected to a node of an intermediate circuit of the power supply system, to which node the load is also connected, via a power switching apparatus.
In this method, connection (coupling) of a respective secondary energy source is preceded by the current drawn by the load or the power drawn by the load being regulated such that the voltage that is present at the node of the intermediate circuit corresponds to the no-load voltage of the secondary energy source that is to be connected or is situated in a prescribed voltage range around the no-load voltage. When the voltages correspond to one another or when the voltage that is present at the node is situated in the prescribed voltage range, the secondary energy source is electrically conductively connected to the node via the power switching apparatus. Subsequently, the current required by the load or the power required by the load is regulated to the relevant value, the load being fed from the primary and secondary energy sources.
Additionally, a method for decoupling at least one secondary energy source from a power supply system, particularly a vehicle onboard power supply system, to which not only the at least one secondary energy source but also a load and a primary energy source having a first voltage characteristic are connected, is provided. The at least one secondary energy source has a respective second voltage characteristic that has an operating voltage range that overlaps the first voltage characteristic. The primary energy source and the at least one secondary energy source are connected to a node of an intermediate circuit of the power supply system, to which node the load is also connected, via a power switching apparatus.
Disconnection of a respective secondary energy source is preceded by the current drawn by the load or the power drawn by the load being regulated such that the voltage that is present at the node of the intermediate circuit corresponds to the no-load voltage of the secondary energy source that is to be disconnected (decoupled) or is situated in a prescribed voltage range around the no-load voltage. When the voltages correspond to one another or when the voltage that is present at the node is situated in the prescribed voltage range, the secondary energy source is electrically isolated from the node via the power switching apparatus. The current required by the load or the power required by the load is then regulated to the relevant value, the load then being fed exclusively from the primary energy source.
This approach is based on the consideration that instead of the regulation of a current regulator that has been performed hitherto, the load supplied with power by the primary energy source is conditioned in the switching process to the extent that its voltage level is identical to the rated no-load voltage of the secondary energy source or is situated in a prescribed voltage range around the no-load voltage and hence can be connected and disconnected without any high compensating or switching currents arising. The risk of destruction of the load connected to the intermediate circuit or of the power switching apparatus is thereby eliminated. The principle can be applied to various switching processes and allows direct coupling of different energy sources by means of the power switching apparatus. The power switching apparatus, which comprises controllable switching arrangements that are each associated with the primary energy source and the at least one secondary energy source, connects the energy sources to one another directly in this case, i.e. connects the energy sources directly to the node of the intermediate circuit or isolates them from the node of the intermediate circuit.
In comparison with a conventional DC/DC converter, the power switching apparatus that is necessary for realizing the method is of much simpler design and, above all, operates almost with no losses. The power switching apparatus can be formed from a number of switching elements that allow a respective energy source to be electrically connected to or blocked from the node of the intermediate circuit.
The invention also provides a power supply system, particularly a vehicle onboard power supply system. The power supply system comprises a load, a primary energy source having a first voltage characteristic and at least one secondary energy source, wherein the at least one secondary energy source has a respective second voltage characteristic that has an operating voltage range that overlaps the first voltage characteristic. The primary energy source and the at least one secondary energy source can be connected to a node of an intermediate circuit of the power supply system, to which node the load is also connected, via a power switching apparatus. In addition, the power supply system comprises a unit for regulating the current drawn by the load or the power drawn by the load.
In a power supply system in such a form, the losses for coupling a plurality of energy sources that can be selectively connected to an intermediate circuit can be kept very low. Furthermore, the large and expensive DC/DC converter used hitherto can be eliminated, allowing costs, weight and installation space to be saved. Similarly, a power switching apparatus that has switching elements can increase the fail-safety of the power supply system.
According to one expedient embodiment of the method, the load is regulated over such a short period of time that a load interruption is imperceptible. In a vehicle, for example, this means that the load is varied completely or just in part for a very short time in an order of magnitude of less than 100 ms, particularly less than 10 ms. By analogy, this corresponds to operation of a clutch in a conventional, manual gearbox of an internal combustion engine with the difference that the load variation can be performed very much more quickly and this is therefore imperceptible to the occupants of the vehicle.
According to a further expedient embodiment, measurement of a first voltage is performed at the terminals of the primary energy source or at other suitable points and measurement of a second voltage is performed at the terminals of the at least one secondary energy source or at other suitable points. The load regulation matches the voltage of the primary energy source to the no-load voltage of the secondary energy source that is to be switched. As soon as the two measured voltages correspond to one another, the secondary energy source is switched by way of the load switching apparatus.
As is already clear from the description above, the load regulation is realized by influencing the operating point of the load.
The power supply system is also distinguished in that its power switching arrangements of the primary energy source and the at least one secondary energy source each comprise associated controllable switching arrangements. The switching arrangements can each include at least one semiconductor switching element or a contactor. Other switching elements can also be used in principle. The choice and embodiment of the respective switching arrangements is dependent on the associated energy source. By way of example, some energy sources may require the avoidance of a current flowing into them in order to avoid damage or an uncontrolled state. This means that some switching elements are preferred or used in a particular interconnection, while other energy sources have no particular precautions or requirements for the switching element(s) used.
By way of example, the primary energy source may be a fuel cell or a rechargeable battery. The secondary energy source may be a rechargeable battery, a solar module, a fuel cell and the like.
The load may be an actuating element, particularly a DC/DC converter or an AC/DC converter, with at least one consumer being connected to the actuating element. By way of example, the consumer may be an electric machine, which can be operated by motor or by generator, or another consumer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.